Scandium is extremely valuable as an additive for high-strength alloys and an electrode material for fuel cells. However, scandium has not yet been used widely due to the small production quantity and high cost thereof.
Meanwhile, a trace amount of scandium is contained in nickel oxide ore such as laterite ore and limonite ore. However, nickel oxide ore has not been industrially used as a raw material for nickel for many years because the grade of nickel contained in nickel oxide ore is low. Consequently, only few studies have been conducted to industrially recover scandium from nickel oxide ore.
However, in recent years, the high pressure acid leach (HPAL) process has been emerging as a practical method, in which nickel oxide ore is introduced into a pressure vessel along with sulfuric acid, and heated at a high temperature of about 240° C. to about 260° C. to allow solid-liquid separation into a leachate containing nickel and a leach residue. In the HPAL process, a neutralizing agent is added to the leachate obtained to separate impurities, and then a sulfurizing agent is added to the resulting leachate from which impurities are separated out, allowing recovery of nickel as nickel sulfide. Subsequently, this nickel sulfide may be subjected to a known nickel refinement process to obtain electrolytic nickel and nickel salt compounds.
In the case of using the HPAL process as described above, scandium contained in nickel oxide ore is contained in a leachate along with nickel (see Patent Document 1). Subsequently, when a neutralizing agent is added to a leachate obtained from the HPAL process to separate impurities, and a sulfurizing agent is then added, nickel is recovered as nickel sulfide while scandium remains in the acidic solution after addition of the sulfurizing agent. In this way, nickel can effectively be separated from scandium by using the HPAL process.
There is also a method in which separation of scandium is performed using a chelating resin (see Patent Document 2). Specifically, in this method disclosed in Patent Document 2, nickel-containing oxide ore is first treated at high temperature and high pressure under an oxidizing atmosphere to selectively leach nickel and scandium into an acidic aqueous solution and an acidic solution is obtained. Subsequently, the pH of the acidic solution is adjusted to the range of 2 to 4, and nickel is then selectively precipitated and recovered as a sulfide using a sulfurizing agent. Next, the resulting solution from which nickel has been recovered is brought into contact with a chelating resin to adsorb scandium to the chelating resin, the chelating resin is washed with a dilute acid, and then the chelating resin after washing is brought into contact with a strong acid to elute scandium from the chelating resin.
Further, as a method for recovering scandium from the acidic solution described above, the method for recovering scandium by means of solvent extraction has also been proposed (see Patent Documents 3 and 4). Specifically, in this method disclosed in Patent Document 3, an organic solvent, in which 2-ethylhexyl sulfonic acid-mono-2-ethylhexyl is diluted with kerosene, is first added to a scandium-containing solution of an aqueous phase, which contains one or more of at least iron, aluminum, calcium, yttrium, manganese, chromium, and magnesium in addition to scandium, to extract a scandium component into the organic solvent. Subsequently, in order to separate yttrium, iron, manganese, chromium, magnesium, aluminum, and calcium extracted into the organic solvent along with scandium, an aqueous solution of hydrochloric acid is added to the organic solvent and scrubbing is performed to remove these. Then, an aqueous solution of NaOH is added to the organic solvent to transform scandium remaining in the organic solvent into a slurry containing Sc(OH)3, and this slurry is filtered to obtain Sc(OH)3, which is then dissolved in hydrochloric acid to obtain an aqueous solution of scandium chloride. Then, oxalic acid is added to the resulting aqueous solution of scandium chloride to obtain a precipitate of scandium oxalate. This precipitate is filtered to separate iron, manganese, chromium, magnesium, aluminum, and calcium into a filtrate, and then calcination is performed to obtain high purity scandium oxide.
Moreover, Patent Document 4 describes a method of selectively separating and recovering scandium from a scandium-containing supply liquid, the method including: bringing the scandium-containing supply liquid into contact with an extracting agent at a certain ratio in a batch process.
The grade of scandium recovered according to these methods is known to be about 95% to 98% pure in terms of scandium oxide. The above grade may be good enough for those uses such as an additive in alloys. However, a much higher purity, for example, the purity of about 99.9%, is required as a grade used for electrolytes of fuel cells which have recently much in demand. Otherwise, their full capability may not be obtained.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H03-173725    Patent Document 2: Japanese Unexamined Patent Application, Publication No. H09-194211    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H09-291320    Patent Document 4: PCT International Publication No. WO2014/110216